A video production switcher includes means for switching between two or more input video signals in order to generate an output video signal. In the case of an abrupt change from one input video signal to another, a simple switch can be used to effectuate the change, but in the case of a slower transition, such as a wipe or a fade, a rotary potentiometer is used to generate a control signal that causes the change to take place over a perceptible time, for example, 0.5 s. In such a case, in order to avoid jerkiness in the transition, and assuming that the potentiometer has a linear angular position to electrical resistance relationship, it is necessary that the shaft of the potentiometer be rotated so that its angular position changes smoothly.
It has been found that a lever arm is more favorable for use under these conditions than a simple rotary knob, because it is easier for the operator to maintain smoothness of motion when the motion is a linear motion of the lower arm than when it is a twisting of the wrist. Nevertheless, in order to maintain smoothness of the motion it is necessary to include a mechanical resistance element in the actuator mechanism so that the operator can, by tactile feedback, avoid jerkiness and irregularities in the movement of the lever arm. It has hitherto been conventional to use discs of synthetic friction material sandwiched between spring-loaded metal plates to provide this tactile feedback, but this is subject to several disadvantages, such as high cost of manufacture and assembly, and a large number of different parts.
An additional problem with use of a lever arm to drive a rotary potentiometer is that axial loading on the shaft of the potentiometer tends to damage the potentiometer and to cause irregularities in the resistance of the potentiometer, regardless of how smoothly the shaft is rotated.